Casting of refractory metals



p 1959 J. B. BRENNAN Q 2,903,759

CASTING 01 REFRACTORY METALS Filed July 6, 1954 2 Sheets-Sheet 1 IN VE/VTOR JOSEPH E. BRENNAN ATTORNE' Y5 A p 1959 .1. B. BRENNAN CASTING OFREFRACTQRY METALS 2 Sheets-Sheet 2 Filed July 6, 1954 JOSEPH B. BRENNANBYyMMM/ 3 2M United States Patent 7 2,903,759 CASTING. on REFRACTORYMETALs Joseph B..Brennan, Cl'eveland,..0hjio;.Helen E Brennan, executrixof the estate of' said Joseph B. Brennan, deceased Applicationluly 6,1954, Serial No. 441,415-

15 Claims. (Cl. 22.-73)j The present invention relates generallytozcasting metals,

2303359 Pate'nteol Sept. 1 5, 1 959 metal from the pools may be fed intothe desired comstantially along the line 3-3 ofFigure l.

and more specifically to'a casting method and apparatus for shapingrefractory; metals including group IV of the periodic table'ofelements,such'as titanium and its alloys.

In casting titanium, titanium alloys, and other highly refractory metalshaving ameltingpoint'inexcessof 1500 or l800 C. a primary problem is toprevent reaction for preheatingthesurface of the cavity to a temperaturesufliciently highto de-gas the mold to prevent blowing, of the casting,said meansxalsobeingeffective to reduce sharply the temperature. of thehigh melting refractory metal poured into the cavity.

Another objectof .the'invention'is: the provision of a method forcasting in. which a predetermined quantity Figure 4 is a cross-sectionalview similar to Figure 2 showing? a modified form of mold.

Figure 5 is aview similar. to Figure 2 in which the mold cavity: has aspecial coating.

Figure 6 is a vertical cross-sectional diagrammatic view illustratingasmeltingiand casting apparatus through which the casting? molds shownin Figures .1 through 4 may be advaneedfor'preheating, casting andcooling.

Eigure-Tis a verticalcross-sectionaldiagrammatic view illustratingahsmelting and casting apparatus wherein metal slugs or sinterings orpowder may be placed above or: intacavitiedlmold member and for shapingby subsequentzmeltingi' Figure. Talso illustratesthe use ofalternatenmolds and sealing: plugs for sealing the vacuum chamber; oftheimold guide or sleeve.

I Figure; 8 is a vertical cross-sectional diagrammatic view ofithechighfrequency coil 148 shownin Figure 7 wherein the sealing. plug has beenmodified to include a cavity for containingcthelmassofmetalto'be melted,evacuated and cast:

of solid refractory metal to be shaped isplacediin a-mold I cavity,heated above its melting point tofill thercavity, and then rapidlycooledunder controlled conditions to solidify the metal.

Another object of the invention is to-provide a mold having relativelythinrefractory walls and contained within a thin-walled flask adapted.to. befilled with molten material at a temperature. substantially belowthe temperature of the metal cast in the mold, to cool the casting.

A further object is to provide a vacuum casting apparatus comprising anelongated vacuumchamber through whichthe molds are adapted to .pass,similar to pistons in a cylinder, means being provided for sealing thechamber between successive molds and between the side walls of the moldsand the side walls of the. chamber to prevent air from entering thevacuum chamberas the molds- Another feature of this invention is theprovision of a.

casting apparatus in which a succession of pools of. molten metal areplaced in partially or fully encompassing relation to a mold guide, saidguide having openings therethrough which align with corresponding,openings in a compartmented mold passing through said'guide, wherebyReferring now to Figures 1 to 3, the mold shown, identifledgenerallybythe'numeral 10, comprises for example; two thin-wall complementarymating sections or halves: 11 and 12:which' divide a cavity 13 of anydesired shape -:or size: A channel 14 may be provided in the top surfaceof the mold parallel with the parting line to facilitate the degassingof the melt andithe mold cavity through the. open upper end; thereof.The complementary sections ofvthe mold may be mechanically secured ormay be cemented together as showninthe vdrawing'by means of a refractorycement. Each mold section or half maybe formed with a peripheralfiange,. section 11 having a peripheral flange designated as 15, andsection 12 having a similar flange designated" as 16. The space behindthe mold cavity andbetween the. flanges of each section is filled with ahighly conductive material 17 such as aluminum, copper.or' silver, orother good thermal conductor.

Since the melting and casting is preferably carried out within a: vacuumchamber, it is desirable that the heat conductive material 17 beonewhich does not vaporize under the high vacuum as used. The mold sections11 and 12 are formed from a refractory ceramic shell having a thin crosssection, say, for example, less'than one-eighth ineh.. Suitablematerials for forming the shell are graphite,

. zirconia, alumina and silica. All of these materials will withstandforvthe time required, the very high temperature of molten-titanium,which is in the vicinity of 1800" C., and will also withstandextreme'temperature changes without cracking.

The mold of Figure 1 is preferably enclosed in a flask 18 shownin-Figure 2, having a relatively thick bottom 18a andtthin sidewalls18b. Thus, when the thermallyconductive material 17 is raised to atemperature above its melting point it remainsin contact with the thinwalled mold halves 11 and 12, rather than flowing away. The flaskpreferably contains an opening through which liquid coolant such asmolten aluminum metal may be introduced to fill the space surroundingthe mold cavity.

As will appear hereinafter the molds are adapted to fit into aguide.tube or sleeve so that a vacuum may be applied to the mold cavityduring casting. The opening a 180 in the flask may be located so as toalign with a corresponding opening in the guide tube through whichcoolant may be introduced from a molten pool of thermally-conductivematerial. The flask has a smooth exterior surface so that it will slidewithin the guide tube, and may be made from graphite, alumina, zirconiaand similar materials which are capable of resisting very hightemperatures.

Figure 4 shows the mold of Figure 1 enclosed in a tight-fittinglongitudinal sleeve 19, which cooperates with flanges 15 and 16 toenclose the conductive coolant material 17. The sleeve 19 serves thesame purpose as the side walls of the flask 18 and prevents the moltencoolant material from escaping from the space provided therefor behindthe mold cavity. Sleeve 19 has a smooth external surface so that it willslide downward within the guide tube of the casting furnace. This sleevemay be made, for example, from stainless steel or refractory ceramicmaterials mentioned as suitable for the mold and flask.

In any case, there must also be a relatively thick, impervious, easy toseal plug of material such as graphite between or as a part of the moldsso that these impervious plugs at the entrance and exit to the guidetube serve as sealing means to maintain the vacuum within the guidetube.

Other suitable means may be used for completely enclosing the conductivematerial 17. For example, the mold halves 11, 12 may have a refractoryceramic backing panel adhered to the upper edge of the flanges 15, 16with the silica cement. This panel may be of any thickness desired andmay also act as an impervious barrier to vacuum leakage or impairment.It is preferable that a resilient sealing member surrounding the mold atall times be in engagement with the perimeter of this panel so as toprevent leakage.

It should be borne in mind that the conductive material 17 will expandconsiderably when heated and a space, as indicated at 21, should beprovided between the material and the confining sleeve 19 to accommodatethe expan- S1011.

To render the surface of the mold cavity less reactive to moltentitanium and similar refractory metals, it is desirable to coat thecavity with an aluminum oxide film, or other protective coating. Forexample, aluminum metal may be applied to the surface of the cavity 13in the form of foil, or it may be cast or sprayed on the surface of thecavity. The aluminum metal film may then, or prior thereto, be convertedinto aluminum oxide as, for example, in accordance with the process ofmy Patent No. 2,346,658. In Figure the mold of Figure 3 is shown havingsuch a coating, designated by the numeral 20. Nickel and/ or molybdenummay also be used as a coating for the mold cavity. Such coating isgenerally applied in a thickness not greater than 0.005 inch. Thecoating material may form an alloy on the surface of the casting or maycomprise a separate film which later may be removed if it is notdesired.

Another suitable material for coating the surface of the mold cavity toreduce and limit activity is carbon. Carbon may be applied in the formof a film of carbon black dusted onto the surface of the cavity, or byflame depositing or by colloidal coating.

According to one way of casting metals in accordance with my invention,the mold of Figure l is preheated higher temperatures are employed it isnecessary to enclose the mold in flask 18 or sleeve 19 as shown inFigure 4. The molten refractory metal to be cast is poured into thecavity of the preheated mold while a vacuum is being applied to thecavity to prevent inclusion of any air. It is very important that thecasting step be carried out in the absence of air and, if desired,

an inert atmosphere comprising a gas such as, for example, argon orother inert gas may be used. The temperature of the molten metal incontact with the surface of the cavity is immediately reduced as theheat is absorbed by the material 17. Hence, there is little opportunityfor chemical reaction to take place. The mold is then subjected toexternal cooling which causes solidification of the casting and then ofthe molten material 17 Heat is continually conducted away from the moldthrough the mass conductive material 17 until the casting has beencooled to the point where it can be removed from the mold. Thetemperature at which the molten metal is poured, for example, may beupwards of 800 higher than the temperature of the mold surface, andconsequently, the heat flow is from the molten metal to the mold,causing the surface of the casting to solidify almost immediately, andform at least a solid skin on the casting.

A suitable apparatus for carrying out the method of the invention isshown in Figure 6. This apparatus is particularly suitable for lowermelting metals such as steel while the apparatus of Figure 7 isparticularly suitable for refractory metals like titanium. Guide tube30,

. having a cross section the same as that of the mold, is

1 ing excessively and the sealing rings mentioned hereinbelow fromoverheating. A series of circumferential slots 33, located within thecooled zone of the coils 29, are fitted with resilient gaskets orpackings 34 to form a seal with the external surface of the mold, thuspreventing atmospheric air from entering the guide tube 30.

If desired, rings like piston rings may be used in place of gaskets 34,provided they have a slight taper to give a lead to the mold beinginserted. The central portion of the guide tube 30 is enclosed in ahousing 35 wherein the preheating, casting and cooling steps are carriedout. The upper portion of the tube 30 is enclosed in a separate housing36, secured to the top of housing 35. The chambers defined by thehousings 36 and 35 are connected to a vacuum line 40 through pipes 41and 42, respectively. The housing 36 is provided primarily to insure ahigh vacuum within the housing 35 and evacuate the mold completelybefore casting takes place. Air carried into tube 30 between the moldsand within the mold cavities is practically all removed through thevacuum line 41.

The guide tube 30 contains a series of apertures 38 through the wallthereof which permits gas escaping from the molds to flow into thechamber surrounding the tube and be removed through the vacuum lines. Ifdesired, inert gas may be introduced into the housing 35 continuouslyand recirculated to insure an inert atmosphere during casting. O-ring 46serves to seal tube 30 where it joins the housing 36. The gaskets, orO-rings, or packings 34 and 46 may be made from a resilient material,preferably one which is heat resistant, such as silicon r ubber. Heatresistant, impregnated and resilient asbestos packings which arecommercially available are also satisfactory.

The preheating zone within housing 35 includes a high frequency coil 48which surrounds the guide tube 30 and is adapted to operate at afrequency that will rapidly heat the conducting metal 17 within themolds. The central portion of the tube within the housing 35 serves as acasting zone in which a pool of molten metal 50, to be shaped in themold, is provided in an at least partially surrounding crucible 52surrounding the tube 30. The metal is introduced through tube 69. Thecrucible 52 is'ofthe usual'type which ismade from ceramic materialsurrounded by heating means, for example, a high frequency coil 55, sothat the metal 50 is retained molten by the inductive effect of thehighv frequency coil. Water circulated around the high frequency coil 55keeps the coil cool. An opening 57 in the tube 30' communicates with thechannel 14 in the top surface of the evacuated mold so that the moltenmetal to be shaped may flow therethrough into the mold cavity 13. Itwill be seen from the drawing that the molds fit snugly within the tube30 so that the metal cannot flow out of the crucible 52 until such timeas the channel 14 becomes aligned with the aperture 57. The loweriportion of the 'tube within the housing 35 comprises a cooling zone,said zone being equipped with coil 59 surrounding and contacting thetube 30. Water or other suitable cooling fluid is circulatedthrough thecoil 59 to conduct away heat and reduce the temperature of'the castingto the point Where it can be removed from the mold. Chilling the moldsimmediately results in improved physical properties of the casting,particularly where the rate of heat transfer is controlled.

For purposes of keeping the high temperature metal 50 in.moltencondition, radiant heating elements 60, as of graphite or siliconcarbide, with a high frequency coil 62 surrounding them, are provided,for example, just above the surface of the metal 50 in the crucible. Theend of the element 60 is parabolic and focuses the heat centrally on thepool of molten metal. Other suitable heating means may be employed, asfor example, an electric arc or a parabolic silicon carbide resistor. Adetailed description of the casting apparatus may be found in mycopending application Serial No. 406,809, filed January 28, 1954.

In a casting apparatus of the type described, molds eight inches inlength and having cavities of about three cubic inches in volume may bemoved downwardly. through the guide tube 30 at a uniform speed ofsixteen inches per minute. If desired, the molds may be movedintermittently through a distance of two inches, for example, andmaintained in each of the preheating, casting and cooling zones forabout four to five seconds. As soon as the channel 14 of'the moldbecomes aligned with the aperture 57, the molten metal 50 flows. rapidlyinto the evacuated cavity 13, to fill the cavity.

Where the mold is made from two halves, there is a possibility that airmay seep through. the joint where the halves are joined. Therefore, itis desirable to insert solid ceramic plugs 43 between successive moldsto prevent such entry of air, unless, of course, the mold is enclosed ina flask, in which case the bottom of the flask serves as a plug. It ispreferable that the length and number of theseplugs be such that oneplug is always in sealing contact with the O-rings 34 at the entranceand exit of the guide tube to prevent atmospheric leakage.

To further improve the seal against atmospheric leakage a pool of moltensealant material 68 may be pro-' vided within the annular enclosure 37near the lower end of the guide tube 30. The molten material may beglass, lead or another relativelylow temperature melting lubricant, andwill flow through the openings 76'into the space between the molds andthe guide tube. Molten material 68 is supplied through tube. 65,surrounded by. high frequency heating coils 67. Coolingcoils 58 maintainthe lower end of tube 30 in cool condition and preventthevsealantmaterial 68 from escaping. by solidifying it.

As indicated, the externalsurface of the mold or flask is smoothso that.a tight sliding contact is made with the inside surface of the guidetube 30. The finish of. such surface is preferably about 70 microns, orbetter.

In'operation, the air within the cavity 13, and air trappedzbetweenmolds and in the pores of the mold is evacuated within the housing 36.The molds are separated by plugs 43 which prevent air seepage betweenthe mold halves. As the mold 10 passes intothe space surrounded by thehigh frequency coil 48-, the conductive metal 17 is heated to itsmelting point which, in the case of aluminum, is 659 C. The heat isimmediately transferred to the lining of the mold cavity, and iseffective to de-gas the mold, thus removing materials that wouldvolatilize upon filling of the cavity with the high temperature metaland blow the casting. As the mold advances past the casting station,molten metal flows into the cavity through the aperture 57. The weightof the metal causes or assists the molds to move downwardly through theguide tube. Therefore, the walls of the mold cavity are relatively cool,and as soon as the high temperature casting metal contacts the wall itis immediately cooled. It will be noted that before the castingoperation is complete the lower end of the mold 10 is already enteringthe cooling zone surrounded by the coil 59. Thus, cooling of the castingis effected from the bottom up to prevent trapping of any gases thatmight'evolve upon contact with the molten metal. effectively seals offthe tube 30 by cooperation with the gaskets 34. The penultimate mold inthe tube makes sealing contact with the first gasket before thelowermost mold is removed, and is further sealed by molten material 68.A clamping ring (not shown) surrounds the lowest mold to frictionallybrake the advance of the stack of molds in the guide tube.

Obviously, other means may be employed in casting high temperaturemetals in molds of the kind disclosed, although the apparatus describedhas proved to operate very satisfactorily. If desired, means may beprovided to permit draining of molten conductive metal 17 from the moldimmediately after casting and the space vacated filled with a coolerfluid to effect complete solidification of the casting. Suitableapparatus for carrying out the casting process, including theintroduction of a second coolant into the mold, is the rotary indexingmold transfer table disclosed inmy copending application Serial No.406,809, filed January 28, 1954.

It will be apparent to those skilled in the art that the mold 10 may beof one-piece construction, formed over a destructible pattern or model.Such a pattern may be made from a low melting alloy, a thermoplasticresin, or a combustible material. The pattern is melted and poured outafter the mold has set, and in the case of the as a slug or powderbriquette.

combustible pattern, the same is burned out leaving a mold cavity 13 ofthe desired shape and size.

Figures 7 and 8 illustrate additional and somewhat modified apparatusfor smelting and casting which is particularly suitable for shapingmetals of group IV of the periodic table.

In accordance with the method employed in this modification, apredetermined quantity of the metal to be shaped is placed in the moldcavity in solid form, such The mold must have a sleeve or flask,enclosing the hollow space surrounding the thin-walled cavity. The moldis then heated to de-gas the cavity and melt the charge of metaltherein. As it advances within the guide tube, molten coolant metalflows into the space surrounding the cavity when the opening providedfor this purpose becomes aligned with an opening in the guide tubeconnecting to a source of molten metal. In this way the casting iscooled initially with final coolingtaking place as the mold passes acooling coil surrounding the lower'portion of the guide tube.

Referring now to Figure 7, a hollow walled mold enclosed in a flask orsleeve, having a smooth surface exterior 119, is fed down into anembracing tube as of graphite. This mold assembly is referred to as mold110 in this description of Figure 7. The mold 110 may be cast ofmaterial such as finely ground 96% silicon oxide, and has a cavitytherein with treated surface, if desired, to prevent or reduce reactionwith highly reactive refractory metal such as group IV metals, and is incon- The last mold in the tube 30 Mold 110 has one or more openings 115leading to the enclosed chamber 112 surrounding the cavity 113 intowhich the final product, such as a group IV metal, may be melted. Whenthe mold 110 is introduced into the tube 130, or prior to suchintroduction, a slug of metal 116, which it is desired to shape, isplaced within the cavity in the mold 110 and as the mold 110 is moveddown through the tube 130 it is sealingly embraced by surroundingresilient rings 134 in the recesses 133 of the tube 130. The metal maybe in the form of powder or a sintered shape as well as a slug, and theweight thereof is predetermined to fill the mold cavity when melted. Thetube 130 at the entrance section is water cooled by the cooling coilsection 136 closely embracing the tube 130.

Plugs 111, which may be a part of the mold 110 or separate membersshaped to fit the guide tube 130, are inserted alternately with themolds 110 to prevent leakage between the molds as they are progressivelyfed down into the vacuum chamber 135.

The chamber 135 is sealed with O-rings 146 about the tube 130 asdescribed above and serves to evacuate tube 130 through openings 138 bymeans of the pipe 141 which is connected to a source of vacuum. A highfrequency coil 148 is provided surrounding the porous section of thetube 130 so that as the molds 110 and the plugs 111 progress downwardthrough the high frequency coil 148 they become highly heated, and theslug or quantity of powdered metal, or sintered metal 116 within thecavity 113 of the mold 110 immediately melts.

The mold cavity 113 is vented through opening 114 leading to theexterior of the guide tube 130 through openings 138 to assist inevacuation of the mold cavity as the mold is lowered through the highfrequency coil 148. The mold 110 has also a wall opening 115 whichaligns peripherally with an opening 157 in tube 130 so that the coolantmetal 150 will flow by gravity through the opening 157, into the hollowspace 112, surrounding the mold cavity. The at least partiallysurrounding pool of molten metal 150 is fully or partially enclosedwithin a ceramic crucible 152, and is retained molten by the highfrequency coils 148 thereabout in the vacuum chamher 135. The coolingmetal may be aluminum or other good thermal conductor which does notvaporize readily under vacuum. The coolant metal 150 is introduced intothe crucible 152 through a wall of the vacuum chamber 135, preferably inthe form of a solid ingot 150a pushed through a tube-like opening bymeans of a suitable pusher member 161. Water-cooled coils 162 surroundthe tube for the purposes of maintaining the ingot 150a solid until itreaches the zone inside the vacuum chamber where it is heated to aboveits melting point by the high frequency coil 148a. The solid,water-cooled section of the ingot 150a may also be sealed by sealinggaskets or O-rings 133a so as to prevent leakage of vacuum thereabout.The crucible 152 containing the molten metal may be open, or it may besealed on top as shown in the drawing. The crucible 152 may be locatedoutside of the vacuum chamber if desired.

After coolant metal 150 has entered the hollow space 112 of the mold110, solidification of the molten metal 116 in the cavity 113, and alsoof the coolant metal, is effected rapidly and progressively as the moldpasses through the embracing cooling coil 159 surrounding the tube 130below the crucible 152.

A sealant 170, such as lead or glass, may be introduced around the moldas it is moved downward through the tube 130 and below the cooling coil.The at least partially surrounding crucible 171 which contains themolten sealant metal may be heated by the high frequency coil 172, andlead 170 may be fed in by a pusher 173 in the tube 174.- The tube 174has a cooling and solification zone 175 through which water flows toretain the metal 170 in the solid state until it comes within theinfluence of the high frequency coil 172, whereupon it is melted and isfed through the openings 176 in the tube 130 to seal and lubricate theplugs and the molds as they move downward. Other sealing and lubricatingmaterials can be substituted in place of lead, such as thermoplasticresins which are normally solid and which would solidify if by chancethey were sucked upward inside the tube 130 within the embracing coolingcoil 159. The sealing means may be fully or partially Within the vacuumchamber 135, rather than below it as shown in Figure 7.

An additional cooling coil 177 may be placed around the tube 130 in thecooling chamber 137. O-rings 147 within recesses in the wall of the tubeelfect additional sealing and garter spring brake member 178 may be usedto retard the molds and the impervious sections therebetween against thepull of gravity as they exit from the tube at the lower end thereof.When filled with metal, the molds are relatively heavy. Brake 178 may beof any suitable mechanical construction, generally a simple frictiondevice will meet the purpose.

A number of molten material supply chambers or crucibles may be placedabout the guide tube 130, with complementary openings in the tube whichalign with the mold openings. Thus, more than one type of material maybe introduced successively either into the interior of the cavity of themold itself or in the hollow chamber about this cavity, or to act as asealant and lubricant for the moving molds and impervious plugs.

During all times of operation a top as well as a bottom surroundingsealing member is in contact with an axially impervious mold section orplug between the molds. Thus, it is important to provide enough O-ringsat the top and bottom ingress and egress portions of tube 130, toprevent flow of atmospheric air around the molds as they enter and leavethe tube. The O-rings and the entrance and exit vacuum chambers arefluid cooled, and in this way are protected from extremely hightemperatures.

In practicing the invention it is important that the molten coolingfluid melt at a temperature substantially below the melting point of themetal being cast, and be a good heat conductor. Preferably, its massshould be two to three times that of the metal being cast. Under theseconditions the molten cast metal is immediately cooled through the thinwalled mold cavity to reduce to a minimum any reaction that might takeplace between the cast metal and the mold cavity or its lining.

Referring to Figure 8, which is the upper portion of the apparatus ofFigure 7 at the heating coil 148, the normally solid plug 111 has beenmodified to include a cavity and is designated by the numeral 180. Aslug or briquette of metal 116 to be shaped, larger than the mouth ofthe mold cavity 113, may be placed within the cavity of the plug tosupply the cavity 113 of mold 110 with molten metal. As the slug passeswithin the electrical field of coil 148 it is melted in a very fewseconds. For example, a slug weighing four ounces will melt in four toten seconds, when the coil 148 has a power of kilovolt amperes and afrequency of 9600 cycles. The molten metal falls into the cavity 113without splattering into the vacuum chamber since the cavity in the plugand in the mold are sealed off, though gas pervious.

The coils 148 may be enclosed in an active nonconductive ceramicmaterial which insulates the coil and prevents scintillation. Under suchconditions higher frequencies may be employed.

It will be understood that the method of feeding solid metals to thecrucibles surrounding the guide tube and the method of feeding themolten metal to the mold, as shown in the vacuum chamber of Figure 7,can also be used in the apparatus of Figure 6.

Other modifications of my invention will occur to those skilled in theart and it is my intention not to limit the invention other than isnecessitated by the scope of the appended claims.

This applicationisa'continuation in part of my applications Serial No;225',949,-filed May'12, 1951, now Patent No. 2,716,790, issued- Sept. 6,1955, and Serial No. 202,707, filed December 26, 1950, nowabandoned,insofar as they contain common subject matter.

What I claim is:

1-. The method of-casting refractory metal which comprises placing a'predetermined amount of said metal in a mold having a-thirr-walledcavity backed by a hollow enclosed-space, heating said metal to melt itand fill the mold cavity, and introducing molten thermally-conductivematerial at a temperature substantially below the melting point of saidrefractory metal into said hollow space to cool said casting throughsaid thin wall.

2. The method of casting refractory metal which comprises placing apredetermined amount of said metal in a mold having a thin-Walled cavitybacked by a hollow enclosed space, evacuating said cavity andmaintaining said vacuum while heating said metal to melt it and fill themold cavity, and introducing molten thermally-conductive material at atemperature substantially below the melting point of said refractorymetal into said hollow space to cool said casting through said thinwall.

3. The method of casting refractory metal which comprises placing apredetermined amount of said metal in a mold having a thin-walled cavitybacked by a hollow enclosed space, evacuating said cavity andmaintaining said vacuum while passing said mold through a high frequencyelectrical field to melt the metal and fill the mold cavity, andintroducing molten thermally-conductive material at a temperaturesubstantially below the melting point of said refractory metal into saidhollow space to cool said casting through said thin wall.

4. An apparatus for casting metals including in combination an elongatedrefractory guide tube, the interior of which is connected to a source ofvacuum, a series of molds adapted to slide through said tube, said moldsbeing spaced from each other by close fitting disc-like plugs which sealthe tube against air seepage through the molds, heating means associatedwith said tube for melting metal charged into said molds, and coolingmeans spaced from said heating means for cooling said molten metal priorto discharge of the mold from said tube.

5. An apparatus for casting metals including in combination an elongatedrefractory guide tube, the interior of which is connected to a source ofvacuum, a series of molds adapted to slide through said tube, said moldsbeing spaced from each other by close fitting disc-like plugs which sealthe tube against air seepage through the molds, heating means associatedwith said tube for melting metal charged into said molds, means forintroducing molten materials between the walls of said mold and thewalls of said tube to seal the space around the molds, and cooling meansspaced from said heating means for cooling said molten metal prior todischarge of the mold from said sleeve.

6. An apparatus for casting metals including in combination an elongatedrefractory perforated guide tube having its central portion enclosed ina vacuum chamber, a series of molds adapted to slide through said tube,each having an enclosed hollow space surrounding the cavity thereof,heating means associated with said tube for melting a solid charge ofmetal within the cavity of the mold, a receptacle containing moltencooling material joining said tube, filling means for introducing saidmolten cooling material into said hollow space of each mold as itadvances past said receptacle and cooling means for conducting heat fromsaid molten charge through said cooling material.

7. An apparatus for casting metals including in combination an elongatedrefractory perforated guide tube having its central portion enclosed ina vacuum chamber, a series of molds adapted to slide through said tube,each having an enclosed hollow space surrounding the cavity thereof,sealing rings at either end of said tube for making close contactwiththe side walls of said molds to seal the tube against air seepage,heatingmeans associated with said tube for melting a solid charge ofmetal within the cavity of the mold, a receptacle containing moltencooling material joining said tube, filling means for introducing saidmolten cooling material into said hollow space of each mold as itadvances past said receptacle and cooling means for conducting heat fromsaid molten charge through said cooling material.

8. In a casting apparatus, guide means for directing=a mold past-aplurality of stations, a mold disposedin said guide means having arelatively thin ceramic refractory shell backed by a conductive metal,preheating means associated with said guide means adapted to melt theconductive metal and thus preheat the mold shell, casting means adjacentsaid preheating means for pouring molten casting metal into saidpreheated mold and cooling means adjacent said casting means effectiveto solidify the conductive metal within the mold and thus cool the shellto solidify the casting.

9. A method for casting refractory metal which comprises providing amold having a relatively thin refractory shell backed by a conductivemetal, said conductive metal having a melting point substantially lowerthan the melting point of the refractory casting metal, melting saidconductive metal to preheat said mold, pouring molten refractory metalinto said mold, and resolidifying said conductive metal to cool therefractory metal cast within the mold.

10. The mold of claim 8 in which the refractory shell is made from aceramic taken from the group consisting of alumina, silica and zirconiaand the conductive metal is aluminum.

11. The method of claim 9 in which the resolidification of theconductive metal is effected from the bottom of the mold upwardly.

12. The method of continuously casting refractory metal which comprisesproviding a succession of molds having an unshaped mass of saidrefractory metal in the cavities thereof, advancing said molds through ahigh frequency electrical field to degas and melt said metal so that themolten metal will flow by gravity to assume the shape of the moldcavity, and continuing to advance said molds through an embracingcooling zone whereby the shaped metal is solidified.

13. The method of continuously casting refractory metal which comprisesproviding a succession of molds each having a cavity of relatively thinrefractory material surrounded by an enclosed hollow space, placing apredetermined unshaped mass of said metal in each said cavity, advancingsaid molds through a guide tube in which the molds are subjectedsuccessively to a vacuum to evacuate the cavity, a high frequency fieldto degas and melt the metal in the cavity, and a two-stage cooling zonein which first said enclosed hollow space is filled with a moltencoolant material followed by cooling of the entire mold, whereby thecasting is solidified, and removing said molds one at a time from saidguide tube.

14. In a method of casting a heat softenable material which comprisesproviding a guide tube maintained at subatmospheric pressure and asuccession of molds, each slightly spaced from the interior of said tubeand having a cavity for shaping said material, and advancing said moldsthrough said tube past a filling station communieating with said tubefor filling said cavities with said material, the step of sealing theinterior of said evacuated guide tube from ingress of gas from theatmosphere which comprises casting a layer of fluid heat softenablematerial on the outside of the mold to seal the space between the moldand the guide tube.

15. In a method of casting molten refractory metal which comprisesproviding a guide tube maintained at subatmospheric pressure and asuccession of molds, each slightly spaced from the interior of said tubeand having a cavity for shaping said metal, and advancing said 7References Cited in the file of this patent UNITED STATES PATENTS RohnJune 30, 1931 Sherwood et al. Dec. 18, 1934 12 Sherwood et al. Apr. 23,Payne Aug. 10, Jacklin June 26, Kennedy Mar. 17, Davis Dec. 1, Lutz Mar.23, Kohl July 6, Findlay June 7, Brennan Sept. 6, Brennan Aug. 20,

